The present invention relates to an x-ray imaging system, in particular, to a dental x-ray imaging system having x-ray tube preheat control circuitry for measuring the x-ray tube current and producing diagnostic radiation when the current is sensed to be appropriate for producing a constant rate of electrons.
A dental x-ray imaging system customarily has an x-ray tube enclosed in a housing called a tube head. One face of the tube head has an opening through which the primary x-ray beam is projected from the x-ray tube target toward the examination subject. A tubular member, called a cone, is coupled to the tube head axially of the opening to limit the x-ray beam to the proper area on the subject's intraoral region.
Some of the primary factors associated with diagnostic x-ray imaging apparatus include the peak voltage applied to the x-ray tube during exposure, the current forced through the x-ray tube in response to the selected peak voltage, and the time of duration of the exposure. The peak voltage determines the penetrating power of the x-ray beam, while the current determines the intensity of the beam.
Substantially all AC powered dental imaging systems have a preheat period, which allows the current flowing through the tube head filament to heat the filament before applying full voltage across the x-ray tube and permitting full current to flow through it. This stabilizes the electron beam emitted by the filament before the x-ray exposure technically begins. That is, prior to the filament reaching an appropriate temperature for producing usable diagnostic x-rays, a plurality of preheat pulses are produced by applying a reduced kilovoltage potential to the x-ray tube, resulting in a number of pulses of non-usable unstable radiation. Though the exposure technically has not yet begun, radiation is therefore being produced at a reduced kilovoltage peak level and is impacting the patient and the film. This preheat time is often much longer than it needs to be, to be sure the filament is indeed “preheated” (has sufficient current flow). Because the levels of this radiation are so low, it is an insignificant contributor to patient dose or image creation. However, this radiation was never a problem until digital imaging sensors became available. Being much more sensitive to radiation than film, digital sensors can become saturated from radiation from the dental x-ray machines due to the radiation provided during this preheat period, accumulated with the radiation during the exposure.
In order to solve the preheating or stabilization problem, prior art designs would either include a separate filament heating circuit to preheat the filament to the appropriate temperature, or provide a self-rectified intraoral x-ray tube design that applies a low level voltage across the x-ray tube for a fixed amount of time to produced the required filament temperature for creating a constant rate of electrons for producing diagnostically useful radiation.
This invention relates to improvements to the structures and methods described above, and to solutions to the problems not solved thereby.